In IC63, it is possible to simultaneously fit the observations in all the photometric bands only if we remove the one at 4.5 \(\mu \) m. This problem has already been encountered in the Horsehead Nebula (see Paper I) and explained therein. Regarding the Orion Bar, we do not have this problem because, whether we use this band or not, we still have a degeneracy between \(a_{\mathrm {min,\,a-C}}\)  and \(\alpha \) . In addition, this does not affect our results on the dust properties. These results confirm what was found for the Horsehead, namely, an increase in the nano-grain minimum size together with a decrease in the nano-grain dust-to-gas mass. For illustrative purposes, the spectra obtained for the Orion Bar are used in [1]} and compared to spectral observations at several representative PDR positions. We also show, in Appendix , a study of the NIR-to-FIR emission ratio across the Orion bar that is purely observation-based and therefore not model-dependent, and which is in agreement with a decrease in the nano-grain abundance when one moves away from the illuminating star.
